Microelectronic imagers are used in a multitude of electronic devices. As microelectronic imagers have decreased in size and improvements have been made with respect to image quality and resolution, they have become commonplace devices and are used in mobile telephones and personal digital assistants (PDAs) in addition to their traditional uses in digital cameras.
Microelectronic imagers include image sensors that typically use charged coupled device (CCD) systems and complementary metal-oxide semiconductor (CMOS) systems, as well as other solid state imager systems.
As shown in FIG. 1, microelectronic imager modules 150 are often fabricated at a wafer level. The imager module 150 includes an imager die 108, which includes an imager array 106 and associated circuits (not shown). The imager array 106 may be a CCD or CMOS imager pixel array, or any other type of solid state imager array. The imager module 150 may also include a lens structure 112, having a spacer 109 and at least one lens element 111 arranged on a lens wafer 510. Spacer 109 maintains the lens element 111 at a proper distance from the imager array 106, such that light striking the lens element 111 is directed appropriately to the imager array 106. The spacer 109 may be bonded to the imager die 108 by a bonding material 104 such as epoxy. Typically, the lens element 111 comprises one or more optically transmissive lenses made of glass or plastic material configured to focus light radiation onto the imager array 106. In addition, the lens structure 112 may be combined with another optically transmissive element, such as a package lid. The fabrication of one such imager module and associated lens support structure is discussed in co-owned U.S. patent application Ser. No. 11/605,131, filed on Nov. 28, 2006 and U.S. patent application Ser. No. 12/073,998, filed on Mar. 12, 2008.
In practice, imager modules 150 are fabricated in mass rather than individually. As shown in a top-down view in FIG. 2A and a cross-sectional view in FIG. 2B, multiple imager dies 108a-108 d, each die including a respective imager array 106a-106d, are fabricated on an imager wafer 90. As shown in FIGS. 3A and 3B, multiple lens elements 111a-111d, corresponding in number and location to the imager arrays 106a-106d on the imager wafer 90 (FIGS. 2A and 2B), may be fabricated on a lens wafer 510 using a replication process such as ultraviolet embossing to duplicate the surface topology of a lens master 480, 485 onto a thin film of an ultraviolet-curable epoxy resin applied to the lens wafer 510. As shown in FIG. 4A, lens wafer 510 is placed so that it is separated from imager wafer 90 by spacers 109. Additionally, lens wafer 510 is located such that lens elements 111a-111d are optically aligned with imager dies 108a-108d to form a plurality of imager modules 150a, 150b (other imager modules are formed, but not shown in FIG. 4A). As shown in FIG. 4B, the imager modules 150a, 150b may then be separated into individual imager modules 150a, 150b by dicing.
One technique for creating convex lens masters 480 necessary for a lens replication process to form multiple lens elements 111a-111d (FIG. 3B) is a jet dispense process. The jet dispense process includes dispensing an appropriate polymer for lens formation onto a glass substrate. Once polymer is applied, a concave lens pin mold 400 is brought from above the polymer and glass substrate to stamp a shape into the polymer. Once the concave lens pin mold 400 is used to shape the polymer, a curing process solidifies the polymer. Once cured, the concave lens pin mold 400 can be removed and the process repeated until the lens master 480 is complete.
The jet dispense process for creating a lens master, however, suffers from certain shortfalls. A first shortfall is that it is difficult to maintain uniform thickness of the lens elements 111a-111d (FIG. 3B) because bonding is done polymer-to-glass. The cured polymer that comprises lens elements 111a-111d is co-extensive with the edges of the lens wafer. Consequently, depending on the uniformity of the jet dispense process, and accuracy of the lens pin mold 400 placement, lens thickness may vary among the edges of the lens master. Any thickness variation is passed on directly to each stamp 300 made from the lens master 480, and ultimately to the lens elements 111a-111d (FIG. 3B) made from the stamp 300. This variation in edge thickness can also be translated to any stacking elements that are bonded to the polymer of the lens elements. Accordingly, a uniform thickness among the lens elements 111 (FIG. 3B)—initiated by a uniform lens master 480—lowers adhesive bond line thickness and makes adhesion of any necessary stacking elements more reliable. Additionally, thickness that is non-uniform may result in chipping or delamination of the polymer at the dicing stage of production, which can lead to decreased image quality.
A second shortfall of the jet dispense process is its comparably low-throughput because each individual lens mold of the lens master 480 must be formed individually. This is necessary in order to ensure uniformity of lenses. The time consuming nature, however, makes it even more essential that the lens master 480 produced—which can be used to make multiple stamps 300—be as close to perfect as possible. Third, jet dispense processes commonly produce residual polymer volume (e.g., sputter) outside the lens area, which can cause problems with formation of other lenses on the lens wafer 510 (FIG. 3B). Fourth, controlling polymer dispense volume is difficult with the jet dispense process and must be precisely maintained for each lens. Fifth, lenses produced by jet dispense processes can have voiding problems as a result of trapped air bubbles. Sixth, accuracy of individual lens alignment on the lens wafer varies directly with the accuracy of the dispensing process. Accordingly, there is a need for a method of fabricating lens masters 480, 485 that yields stamps 300, 305 for forming discrete cured lenses 540, 545 that mitigates against the drawbacks of the jet dispense process.